Oxymethylene copolymer with poly-β-alanine

ABSTRACT

Disclosed is an oxymethylene copolymer resin composition comprising (A) 100 parts by weight of an oxymethylene copolymer resin comprising a plurality of oxymethylene copolymer chains, each comprising recurring oxymethylene monomer units and oxyalkylene monomer units inserted therein, wherein the oxyalkylene monomer units, each having at least 2 carbon atoms, are present in the oxymethylene copolymer resin in an amount of from 0.05 to 0.5 mol %, based on the oxymethylene monomer units, the plurality of oxymethylene copolymer chains collectively having, as terminal groups, alkoxyl groups each having at least one carbon atom, hydroxyalkyl groups each having at least 2 carbon atoms, and formate groups; (B) 0.01 to 3.0 parts by weight of a poly-β-alanine having an average particle diameter of 6 μm or less; and (C) from 0.001 to 0.6 part by weight of an alkaline earth metal salt of a fatty acid, an amino-substituted triazine or a hydrotalcite.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an oxymethylene copolymer resincomposition. More particularly, the present invention is concerned withan oxymethylene copolymer resin composition comprising an oxymethylenecopolymer resin and a poly-β-alanine, wherein the oxymethylene copolymerresin comprises a plurality of oxymethylene copolymer chains, eachcomprising recurring oxymethylene monomer units and oxyalkylene monomerunits inserted therein in a specific ratio, wherein the oxymethylenecopolymer chains have specific terminal groups, and wherein thepoly-β-alanine is in a finely pulverized form. The oxymethylenecopolymer resin composition of the present invention exhibits not onlymechanical properties which are comparable or superior to the mechanicalproperties of conventional oxymethylene polymer compositions as well asconventional oxymethylene polymers, but also exhibits an excellentthermal stability which has not conventionally been achieved. Theconventional oxymethylene polymers have serious disadvantages in thatthe unstable terminals of the oxymethylene polymer chains aresusceptible to heat decomposition to form formaldehyde, and the formedformaldehyde is likely to be oxidized to thereby form formic acid. Theformic acid thus formed promotes a decomposition of the main chain ofthe oxymethylene polymer. However, in the oxymethylene copolymer resincomposition of the present invention, not only can the formation offormaldehyde which is likely to be unfavorably caused by a heatdecomposition of the unstable terminals of the copolymer chains besuppressed due to the oxyalkylene monomer units inserted in therecurring oxymethylene monomer units in a specific ratio, but also thefinely pulverized poly-β-alanine contained in the compositionefficiently captures formaldehyde which is still likely to be generateddue to the inherent difficulty in complete suppression of the formationof formaldehyde, so that the formation of formic acid (which promotes adecomposition of the main chain of the oxymethylene copolymer) can beeffectively suppressed. Therefore, the thermal stability of theoxymethylene copolymer resin composition of the present invention isextremely high under oxygen-containing atmosphere conditions as comparedto the thermal stability of the conventional oxymethylene polymercompositions as well as the conventional oxymethylene polymers.

2. Background Art

Conventionally, oxymethylene homopolymers have been widely used asmaterials for automobile parts, electrical parts, etc., since theoxymethylene homopolymers have not only a good balance of mechanicalproperties but also excellent fatigue resistance properties. However,the thermal stability of the conventional oxymethylene homopolymerduring the molding thereof is unsatisfactory. This is because theterminal acetyl groups of the oxymethylene homopolymer are likely to beeliminated from the main chain of the homopolymer by heating, and theamount of the formed formaldehyde is increased in accordance with theadvance of the decomposition of the terminals of the homopolymer. When alarge amount of formaldehyde is formed, disadvantages occur such thatthe homopolymer is likely to suffer foaming during the molding thereofand that traces of escape of gaseous formaldehyde are left in thesurface of the resultant shaped article to cause the surface appearanceto be poor. Further, the formed formaldehyde is oxidized by oxygenpresent in the molding machine to thereby form formic acid, and theformed formic acid promotes the decomposition of the main chain of theoxymethylene homopolymer.

Unexamined Japanese Patent Application Laid-Open Specification No.5-5017 discloses an oxymethylene copolymer exhibiting not onlymechanical properties which are comparable to the mechanical propertiesof an oxymethylene homopolymer, but also an improved thermal stability.In this prior art document, a small amount of oxyalkylene monomer isinserted in the polymer chain of the oxymethylene homopolymer, and theamount of terminal formate groups is decreased. However, although thethermal stability of the oxymethylene copolymer in a nitrogen atmosphereis improved, the thermal stability under oxygen-containing atmosphereconditions is still poor.

Unexamined Japanese Patent Application Laid-Open Specification No.2-247247 discloses an oxymethylene polymer composition comprising anoxymethylene polymer and a poly-β-alanine. In this composition, when theoxymethylene polymer is an oxymethylene homopolymer, the terminal acetylgroups are eliminated by heating in the molding of the composition, andthe decomposition of the terminals of the polymer advances, so that thethermal stability of the composition under oxygen-containing atmosphereconditions is poor. On the other hand, when the oxymethylene polymer isa commercially available oxymethylene copolymer, the mechanicalproperties of the composition are poor.

SUMMARY OF THE INVENTION

In these situations, the present inventors have made extensive andintensive studies with a view toward developing an oxymethylenecopolymer resin composition having not only mechanical properties whichare comparable or superior to the mechanical properties of theconventional oxymethylene polymer compositions as well as theconventional oxymethylene polymers, but also an excellent thermalstability in an oxygen-containing atmosphere as compared to the thermalstability of the conventional oxymethylene polymer compositions as wellas the conventional oxymethylene polymers. As a result, it hasunexpectedly been found that, with respect to an oxymethylene copolymerresin composition comprising an oxymethylene copolymer resin and apoly-β-alanine, wherein the oxymethylene copolymer resin comprises aplurality of oxymethylene copolymer chains, each comprising recurringoxymethylene monomer units and oxyalkylene monomer units insertedtherein, when the oxyalkylene monomer units are present in theoxymethylene monomer units in a specific ratio, and the poly-β-alanineis in a finely pulverized form, having an average particle diameter of 6μm or less, an extremely high thermal stability of the composition canbe achieved. Further, the present inventors have made studies on theabove-mentioned unexpected effect of the oxymethylene copolymer resincomposition of the present invention. As a result, it has been foundthat in the composition of the present invention, not only can theformation of formaldehyde which is likely to be unfavorably caused by aheat decomposition of the unstable terminals of the copolymer chains besuppressed due to the oxyalkylene monomer units inserted in therecurring oxymethylene monomer units in a specific ratio, but also thefinely pulverized poly-β-alanine can efficiently capture formaldehydewhich is still likely to be generated due to the inherent difficulty incomplete suppression of the formation of formaldehyde, so that theformation of formic acid (which promotes a decomposition of the mainchain of the oxymethylene copolymer) from the formaldehyde, can beeffectively suppressed and, therefore, the extremely high thermalstability of the oxymethylene copolymer resin composition can beachieved. The present invention has been completed based on thesefindings.

Therefore, it is an object of the present invention to provide anoxymethylene copolymer resin composition exhibiting mechanicalproperties which are comparable or superior to the mechanical propertiesof the conventional oxymethylene polymer compositions as well as theconventional oxymethylene polymers, and an excellent thermal stabilitywhich has not conventionally been achieved.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided an oxymethylenecopolymer resin composition comprising:

(A) 100 parts by weight of an oxymethylene copolymer resin comprising aplurality of oxymethylene copolymer chains, each comprising recurringoxymethylene monomer units and oxyalkylene monomer units insertedtherein, wherein said oxyalkylene monomer units, each having at least 2carbon atoms, are present in the oxymethylene copolymer resin in anamount of from 0.05 to 0.5 mol %, based on the oxymethylene monomerunits,

the plurality of oxymethylene copolymer chains collectively having, asterminal groups, alkoxyl groups each having at least one carbon atom,hydroxyalkyl groups each having at least 2 carbon atoms, and formategroups; and

(B) 0.01 to 3.0 parts by weight of a poly-β-alanine having an averageparticle diameter of 6 μm or less.

In the oxymethylene copolymer resin of the oxymethylene copolymer resincomposition of the present invention, the amount of the oxyalkylenemonomer units inserted in the recurring oxymethylene monomer units isimportant. The amount of the oxyalkylene monomer units inserted in therecurring oxymethylene monomer units is from 0.05 to 0.5 mol %,preferably, from 0.1 to 0.4 mol %, based on the oxymethylene monomerunits. When the amount of the oxyalkylene monomer units inserted in theoxymethylene monomer units is smaller than 0.05 mol %, the amount offormaldehyde formed by a heat decomposition of the oxymethylene monomerunits in an oxygen-containing atmosphere becomes undesirably large. Inthis case, even when a poly-β-alanine is added to the oxymethylenecopolymer resin, the amount of formaldehyde formed by the heatdecomposition of the oxymethylene monomer units is so large that thepoly-β-alanine cannot satisfactorily capture the formed formaldehyde.Further, since the formed formaldehyde is likely to be oxidized to formformic acid which promotes a decomposition of the main chain of theoxymethylene copolymer in an oxygen-containing atmosphere, the thermalstability of the resin composition becomes poorer. On the other hand,when the amount of the oxyalkylene monomer units is larger than 0.5 mol%, the mechanical properties of the resin composition are markedlylowered.

The amount of the oxyalkylene monomer units inserted in the oxymethylenemonomer units can be determined as follows. The oxymethylene copolymerresin is subjected to heat decomposition in a 3 N aqueous solution ofhydrochloric acid, and the amount of the oxyalkylene monomer units isdetermined by analyzing alkylene glycol formed in the resultant aqueoussolution.

With respect to the terminal groups of the oxymethylene copolymer chainin the resin composition of the present invention, explanation is madebelow.

In the present invention, the plurality of the oxymethylene copolymerchains collectively have, as terminal groups, alkoxyl groups (such asmethoxyl group), hydroxyalkyl groups (such as hydroxyethyl group) andformate groups.

The terminal alkoxyl groups having at least one carbon atom are derivedfrom a formal which is used as a molecular weight modifier in thecopolymerization described below. For example, methylal (CH₃ O)₂ CH₂ !is generally used as a molecular weight modifier. In this instance,methoxyl groups are formed as terminal groups. There is no particularlimitation with respect to the number of carbon atoms of each terminalalkoxyl group. However, from the viewpoint of ease in synthesis andpurification of a formal as a molecular weight modifier, each terminalalkoxyl group independently has preferably 1 to 10 carbon atoms, morepreferably 1 to 3 carbon atoms.

The terminal hydroxyalkyl groups, such as hydroxyethyl group (--CH₂ CH₂OH) and hydroxybutyl group, are derived from a cyclic ether or cyclicformal, which is one of the raw materials for producing the oxymethylenecopolymer. The terminal hydroxyalkyl groups are formed as follows.During the production of an oxymethylene copolymer resin comprisingrecurring oxymethylene monomer units and oxyalkylene monomer units(which are derived from a cyclic ether or cyclic formal) insertedtherein is produced, due to the presence of a small amount of water inraw materials for producing the oxymethylene copolymer, hydroxymethylgroups (which are unstable under heated conditions) are inevitablyformed as terminal groups. As described below in detail, when theproduced oxymethylene copolymer resin having unstable terminalhydroxymethyl groups is subjected to post-treatment, for example, heattreatment in an aqueous solution of an alkaline substance (such astriethylamine), the unstable terminals (having terminal hydroxymethylgroups) of the oxymethylene copolymer chains are decomposed. Thisdecomposition of the terminals of the copolymer chains proceeds towardthe inner portion of the main chain of the copolymer, and thedecomposition terminates at a site where an oxyalkylene monomer unit islocated, so that the oxyalkylene monomer unit at such a site is changedto a stable terminal hydroxyalkyl group, such as hydroxyethyl group,hydroxybutyl group or the like. There is no particular limitation withrespect to the number of carbon atoms of each terminal hydroxyalkylgroup. However, generally, each terminal hydroxyalkyl group has at leasttwo carbon atoms. From the viewpoint of each in synthesis andpurification of a cyclic ether and cyclic formal as raw materials, it ispreferred that each terminal hydroxyalkyl group independently have 2 to10 carbon atoms. When the oxymethylene copolymer chain has a terminalhydroxymethyl group, the terminal hydroxymethyl group is eliminated fromthe copolymer chain by heat during the molding, so that thedecomposition of the terminals of the copolymer chain proceeds whileforming formaldehyde. When the above-mentioned unstable terminalhydroxymethyl groups are largely present in the oxymethylene copolymerresin, a large amount of formaldehyde is necessarily formed by the heatdecomposition, so that even if a poly-β-alanine is added to theoxymethylene copolymer resin, the amount of the formaldehyde, whichcannot be captured by the poly-β-alanine, becomes large. Further, theformed formaldehyde reacts with oxygen present in the molding machine tothereby produce formic acid (which promotes the decomposition of themain chain of the oxymethylene copolymer), so that the thermal stabilityof the oxymethylene copolymer resin composition becomes poor. In thepresent invention, it is preferred that the unstable terminals havingterminal hydroxymethyl groups are changed to stable terminals to adegree such that the oxymethylene copolymer resin exhibits a weightdecrease ratio R of 3% or less as measured by heating the oxymethylenecopolymer resin at a temperature of 230° C. for 100 minutes in anitrogen atmosphere. The above-mentioned weight decrease ratio R isdefined by the following formula: ##EQU1## wherein W₁ is the weight ofthe oxymethylene copolymer resin before the heating, and W₂ is theweight of the oxymethylene copolymer resin after the heating.

The terminal formate groups (--OOCH) are formed by the decomposition ofa main chain of the oxymethylene copolymer, which is caused by a hydrideshift reaction (see, for example, H. D. Herman, E. Fisher, K.Weissermel, Macromol Chem., 90, p. 1, 1966) which occurs as a sidereaction during the copolymerization. The ratio of the terminal formategroups can be expressed in terms of the absorbance ratio D₁₇₁₀ /D₁₄₇₀ inthe infrared absorption spectrum, wherein the absorbance D₁₇₁₀ at thewave number of 1710 cm⁻¹ is ascribed to the terminal formate groups andthe absorbance D₁₄₇₀ at the wave number of 1470 cm⁻¹ is ascribed to theoxymethylene groups. Each of absorbances D₁₇₁₀ and D₁₄₇₀ of theoxymethylene copolymer can be obtained by a method in which theoxymethylene copolymer resin (A) is hot-pressed to obtain a film, and aninfrared absorption spectrum was taken with respect to the film. It ispreferred that the amount of the terminal formate groups in theoxymethylene copolymer resin be in an amount such that a relationshipdefined by the formula D₁₇₁₀ /D₁₄₇₀ ≦0.025 can be satisfied, morepreferably in an amount such that a relationship defined by the formulaD₁₇₁₀ /D₁₄₇₀ ≦0.02 can be satisfied. When the D₁₇₁₀ /D₁₄₇₀ ratio is morethan 0.025, formaldehyde is formed in a large amount by a heatdecomposition of the terminals of the oxymethylene copolymer chains(which heat decomposition is caused by the elimination of the terminalformate groups), so that even if a poly-β-alanine is added to theoxymethylene copolymer resin, the amount of the formaldehyde, whichcannot be captured by the poly-β-alanine, becomes large. Further, asmentioned above, the formed formaldehyde is oxidized with oxygen presentin the molding machine to thereby form formic acid (which promotes thedecomposition of the main chain of the oxymethylene copolymer), so thatthe thermal stability of the oxymethylene copolymer resin compositionbecomes poor.

Hereinbelow, explanation is made with respect to the method forproducing the oxymethylene copolymer resin composition of the presentinvention.

For producing the oxymethylene copolymer resin to be used in the presentinvention, formaldehyde or trioxane is copolymerized with a cyclic etheror a cyclic formal using a cation polymerization catalyst. The cyclicether to be used in the present invention is represented by formula (I):##STR1## wherein each of R¹ and R² independently represents a hydrogenatom, a C₁ -C₄ alkyl group or a C₆ -C₁₂ aryl group, and m represents aninteger of from 2 to 6.

The cyclic formal to be used in the present invention is represented byformula (II): ##STR2## wherein each of R³ and R⁴ independentlyrepresents a hydrogen atom, a C₁ -C₄ alkyl group or a C₆ -C₁₂ arylgroup, and m represents an integer of from 2 to 6.

Examples of cyclic ethers represented by formula (I) above includeethylene oxide, propylene oxide, butylene oxide and styrene oxide. Ofthese, ethylene oxide is especially preferred.

Examples of cyclic formals represented by formula (II) above includeethylene glycol formal (1,3-dioxolane), diethylene glycol formal,1,3-propanediol formal, 1,4-butanediol formal, 1,5-pentanediol formaland 1,6-hexanediol formal. Of these, ethylene glycol formal(1,3-dioxolane) and 1,4-butanediol formal are especially preferred.

The number of carbon atoms of each of the cyclic ether and cyclic formalis not particularly limited. However, from the viewpoint of ease insynthesis and purification of a cyclic ether and cyclic formal, it ispreferred that the cyclic ether have 2 to 10 carbon atoms and that thecyclic formal have 3 to 11 carbon atoms.

The above-mentioned cyclic ethers and cyclic formals may be usedindividually or in combination.

In the present invention, when formaldehyde is used as a raw materialfor forming the oxymethylene monomer units, the cyclic ether or cyclicformal is used in an amount of from 0.05 to 0.8 mol %, based on theformaldehyde, and when trioxane is used as a raw material for formingthe oxymethylene monomer units, the cyclic ether or cyclic formal isused in an amount of from 0.15 to 2.5 mol %, based on the trioxane.

Examples of cation polymerization catalysts which are used for obtainingthe oxymethylene copolymer resin to be used in the present inventioninclude Lewis acids, such as boron trifluoride, tin tetrachloride,titanium tetrachloride, phosphorus pentafluoride and phosphoruspentachloride, and complexes or salts thereof; and superstrong acids,such as trifluoromethanesulfonic acid, pentafluoroethanesulfonic acid,heptafluoropropanesulfonic acid, nonafluorobutanesulfonic acid,undecafluoropentanesulfonic acid and perfluoroheptanesulfonic acid, andderivatives thereof. Specific examples of complexes of boron trifluorideinclude boron trifluoride dibutyl ether. Examples of derivatives ofsuperstrong acids include anhydrides, alkyl esters and alkyl silylesters of superstrong acids. Specific examples of superstrong acidanhydrides include trifluoromethanesulfonic acid anhydride,pentafluoroethanesulfonic acid anhydride, heptafluoropropanesulfonicacid anhydride, nonafluorobutanesulfonic acid anhydride,undecafluoropentanesulfonic acid anhydride and perfluoroheptanesulfonicacid anhydride. Specific examples of alkyl esters of superstrong acidsinclude methyl trifluoromethanesulfonate, ethyltrifluoromethanesulfonate, methyl pentafluoroethanesulfonate and methylheptafluoropropanesulfonate. Specific examples of alkyl silyl esters ofsuperstrong acids include trimethylsilyl trifluoromethanesulfonate andtrietylsilyl trifluoromethanesulfonate. Of the above cationpolymerization catalysts, boron trifluoride, complexes of borontrifluoride (e.g., boron trifluoride dibutyl ether), andtrifluoromethanesulfonic acid and a derivative thereof, are especiallypreferred.

With respect to the concentration of the cation polymerization catalyst,when the catalyst is, for example, boron trifluoride or a complexthereof, the catalyst concentration is preferably 5×10⁻⁶ to 5×10⁻⁵ mol,more preferably 0.5×10⁻⁵ to 4×10⁻⁵ mol per mol of the total of the rawmaterials, i.e., formaldehyde or trioxane and a cyclic ether or a cyclicformal. On the other hand, when the catalyst is trifluorosulfonic acidor a derivative thereof, the catalyst concentration is preferably 1×10⁻⁸to 5×10⁻⁷ mol, more preferably 3×10⁻⁸ to 5×10⁻⁸ mol per mol of the totalof the raw materials, i.e., formaldehyde or trioxane and a cyclic etheror a cyclic formal. When the catalyst concentration is lower than theabove-mentioned range, the yield of the oxymethylene copolymer resinbecomes low. On the other hand, when the catalyst concentration ishigher than the above-mentioned range, the oxymethylene copolymer resinundergoes acid decomposition by the action of the cation polymerizationcatalyst remaining in the oxymethylene copolymer resin, leading to alowering of the thermal stability of the resin. Further, when theconcentration of the cation polymerization catalyst is too high, thehydride shift reaction is likely to vigorously occur during thepolymerization and hence the amount of the terminal formate groups isincreased, so that D₁₇₁₀ and D₁₄₇₀ cannot satisfy the relationshipdefined by the formula D₁₇₁₀ /D₁₄₇₀ ≦0.025.

With respect to the polymerization apparatus to be used in the presentinvention, there is no particular limitation, and the polymerizationapparatus may be of a batch type or of a continuous type. As a batchtype polymerization apparatus, a reaction vessel having a stirrer cangenerally be used. As a continuous type polymerization apparatus, aself-cleaning type mixer, such as a co-kneader, a twin-screw continuousextrusion kneader or a twin-paddle type continuous mixer, can be used.The polymerization can be conducted at 60 to 200° C., preferably 60 to140° C., under atmospheric pressure. With respect to the time for thepolymerization, there is no particular limitation. However, in general,the polymerization time is in the range from 10 seconds to 100 minutes.After the polymerization, the catalyst remaining in the obtained polymercauses a depolymerization of the polymer. Therefore, the remainingcatalyst is usually deactivated. As a method for the deactivation of acatalyst, there can be generally employed a method in which the catalystis contacted with an aqueous solution of a basic compound, such astriethylamine, or with an organic solvent, thereby neutralizing anddeactivating the catalyst, or a method in which a basic compound issimply added to the reaction mixture, thereby neutralizing anddeactivating the catalyst.

As described above, an oxymethylene copolymer resin as produced in theabove-mentioned manner has an unstable molecular terminal which containsa hydroxymethyl group. The unstable terminal can be changed to a stableterminal by a customary method. For example, an oxymethylene copolymerresin having an unstable molecular terminal can be heat-treated at 180to 250° C. for 30 seconds to 20 minutes together with an alkalinesubstance, such as an aqueous triethylamine solution, to thereby changean unstable terminal hydroxymethyl group to a stable hydroxyalkyl group,such as a hydroxyethyl group or a hydroxybutyl group, thus stabilizingthe molecular terminals. As described above, with respect to thestabilization of molecular terminals, it is preferred that unstableterminals containing hydroxymethyl groups be changed to stable terminalsto a degree such that the oxymethylene copolymer resin exhibits a weightdecrease ratio (R) of 3% or less as measured by heating the oxymethylenecopolymer resin at 230° C. for 100 minutes in a nitrogen atmosphere. Forthis purpose, various measures may be adopted. For example, measuresthat may be adopted include prolonging the time of stabilizationtreatment or increasing the concentration of an alkaline substance usedfor stabilization treatment. As mentioned above, when oxyalkylenemonomer units are present in the oxymethylene copolymer resin in anamount of less than 0.05 mol %, based on the oxymethylene monomer units,the thermal stability of the oxymethylene copolymer resin becomes poor.Therefore, in such a case, the change of unstable molecular terminalscontaining hydroxymethyl groups to stable terminals cannot be conductedto a satisfactory degree due to the poor thermal stability of thecopolymer, so that the oxymethylene copolymer resin cannot exhibit aweight decrease ratio (R) of 3% or less as measured by heating theoxymethylene copolymer resin at 230° C. for 100 minutes in a nitrogenatmosphere.

The oxymethylene copolymer resin composition of the present inventionhas a number average molecular weight of about 3,000 to about 200,000,and has a melt index of about 0.5 to about 200 g/10 minutes as measuredat 190° C. in accordance with ASTM D1238.

In the oxymethylene copolymer resin composition of the presentinvention, as component (B), a finely pulverized poly-β-alanine havingan average particle diameter of 6 μm or less is used. By using such afinely pulverized poly-β-alanine, formaldehyde can be capturedefficiently. As a poly-β-alanine which can be used in the presentinvention, there can be mentioned poly-β-alanines disclosed, forexample, in U.S. Pat. No. 4,855,365, Examined Japanese PatentApplication Publication No. 4-4340, and Unexamined Japanese PatentApplication Laid-Open Specification Nos. 63-118328, 2-251535 and3-234729.

The above-mentioned poly-β-alanine is a copolymer which comprises twodifferent types of recurring acrylamide monomer units of formulae (I)and (II): ##STR3## The amido groups of the poly-β-alanine react withformaldehyde formed by a heat decomposition of the oxymethylenecopolymer chain, thereby capturing the formaldehyde.

Further, the poly-β-alanine may be a copolymer of acrylamide with avinyl group-containing monomer other than acrylamide. Examples of vinylgroup-containing monomers other than acrylamide include n-butylmethacrylate, isobutyl methacrylate, stearyl methacrylate,divinylbenzene, ethylenebisacrylamide and N,N'-methylenebisacrylamide.Of these, N,N'-methylenebisacrylamide is especially preferred.

The poly-β-alanine to be used in the present invention can be obtainedby polymerizing acrylamide, optionally with the above-mentioned vinylgroup-containing monomer, in the presence of a metal alcoholate, such ascalcium alkoxide and zirconium alkoxide.

In the present invention, after a poly-β-alanine is produced by theabove-described method, it is necessary that the poly-β-alanine berendered particulate so as to have an average particle diameter of 6 μmor less. The poly-β-alanine to be used in the present invention isinfusible at the molding temperature of an oxymethylene copolymer resincomposition, so that, during molding, the smaller the average particlediameter of the poly-β-alanine, the larger the surface area of thepoly-β-alanine per unit weight. A poly-β-alanine having a large surfacearea per unit weight can efficiently capture formaldehyde which isformed when an oxymethylene copolymer resin is exposed to heat in anoxygen-containing atmosphere. By the addition of a poly-β-alanine havingan average particle diameter of 6 μm or less, the amount of formaldehyde(which is likely to be oxidized to form formic acid) can be decreasedand hence the formation of formic acid is suppressed, so that thethermal stability of the oxymethylene copolymer resin composition in anoxygen-containing atmosphere is improved. By contrast, when thepoly-β-alanine employed has an average particle diameter of more than 6μm, formaldehyde formed when the oxymethylene copolymer resin is exposedto heat in an oxygen-containing atmosphere cannot be efficientlycaptured, so that a large amount of the poly-μ-alanine is need. When alarge amount of a poly-μ-alanine is used, the oxymethylene copolymerresin composition undergoes discoloration during the molding to assumean unintended color, so that the appearance of the resultant shapedarticle becomes poor. The amount of the poly-β-alanine to be used in thepresent invention is 0.01 to 3.0 parts by weight, preferably 0.1 to 0.5part by weight per 100 parts by weight of the oxymethylene copolymerresin. When the amount of the poly-β-alanine is less than 0.01 part byweight per 100 parts by weight of the oxymethylene copolymer resin, theamount of formaldehyde formed at the time of molding cannot besatisfactorily decreased. On the other hand, when the amount of thepoly-β-alanine is larger than 3.0 parts by weight, as mentioned above,the oxymethylene copolymer resin composition undergoes discolorationduring the molding to assume an unintended color, so that the appearanceof the resultant shaped article becomes poor.

The oxymethylene copolymer resin composition of the present inventionmay further contain a specific basic substance in addition to theoxymethylene copolymer resin and the poly-β-alanine. Such a specificbasic substance is advantageously used to neutralize formic acid (whichis formed in a small amount when the oxymethylene copolymer resincomposition is heated in an oxgen-containing atmosphere), so that thedecomposition of the main chain of the oxymethylene copolymer issuppressed.

Examples of specific basic substances include an alkaline earth metalsalt of an organic fatty acid (having preferably from 5 to 40 carbonatoms, more preferably 8 to 25 carbon atoms), an amino-substitutedtriazine and a hydrotalcite.

Specific examples of organic fatty acids include lauric acid, stearicacid, ricinolic acid, behenic acid, lignoceric acid, carotenic acid,montanic acid and melissic acid, and substitution products of thesefatty acids with a hydroxyl group. Specific examples of alkaline earthmetals include magnesium, calcium and beryllium.

Of the above-mentioned alkaline earth metal salts of organic fattyacids, calcium stearate is especially preferred.

Examples of amino-substituted triazine include guanamine(2,4-diamino-sym-triazine), melamine (2,4,6-triamino-sym-triazine),N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine,N,N-diallylmelamine, N,N',N"-triphenylmelamine, N-methylolmelamine,N,N'-dimethylolmelamine, N,N',N"-trimethylolmelamine, benzoguanamine(2,4-diamino-6-phenyl-sym-triazine), 2,4-diamino-6-methyl-sym-triazine,2,4-diamino-6-butyl-sym-triazine, 2,4-diamino-6-benzyloxy-sym-triazine,2,4-diamino-6-butoxy-sym-triazine,2,4-diamino-6-cyclohexyl-sym-triazine,2,4-diamino-6-chloro-sym-triazine, 2,4-diamino-6-mercapto-sym-triazine,2,4-dioxy-6-amino-sym-triazine, 2-oxy-4,6-diamino-sym-triazine andN,N',N"-tetracyanoethylbenzoguanamine.

Examples of hydrotalcites include a natural hydrotalcite represented bythe formula: Mg₀.75 Al₀.25 (OH)₂ Co₃ 0.125.0.5H₂ O, and a synthesizedhydrotalcite represented by, for example, the formula: Mg₄.5 Al₂ (OH)₁₃Co₃.3.5H₂ O. These hydrotalcites may be those which have been treated toremove water of crystallization thereof.

The above-mentioned basic substance can be incorporated in theoxymethylene copolymer resin composition of the present invention in anamount of 0.001 to 0.6 part by weight, preferably 0.01 to 0.3 part byweight, per 100 parts by weight of the oxymethylene copolymer resin.When the amount of the basic substance is larger than 0.6 part byweight, the oxymethylene copolymer resin composition is likely to sufferdiscoloration during the molding to assume an unintended color, so thatthe appearance of the resultant shaped article becomes poor. On theother hand, when the amount of the basic substance is less than 0.001part by weight, almost no effect of neutralizing formic acid isobtained.

In the composition of the present invention, various additives andagents which are used in conventional resin compositions containing anoxymethylene homopolymer or oxymethylene copolymer, can be incorporated.Examples of such additives and agents include an antioxidant, anultraviolet absorber, a light stabilizer, a lubricant, a mold-releaseagent, a pigment and an inorganic filler. These additives and agents canbe used individually or in combination.

With respect to the method for preparing the composition of the presentinvention, there is no particular limitation. For example, with respectto the form of the basic substance and additives to be added to thecomposition of the present invention, the basic substance and additivesmay be in a powdery form or may be in a molten form. In general, thecomposition can be prepared by charging the oxymethylene copolymerresin, the poly-β-alanine and any additives together into an extruderand kneading the same. The kneader may be of a single-screw type or maybe of a twin-screw type. The extrusion temperature can be appropriatelyselected from the range of 180 to 240° C.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Referential Examples, Examples andComparative Examples, but they should not be construed as limiting thescope of the present invention.

In the Examples and Comparative Examples, various measurements wereconducted by the following methods.

(1) Amount of oxyalkylene monomer units inserted:

10 g of an oxymethylene copolymer resin is placed in 100 ml of anaqueous 3 N HCl solution, and the whole is heated in a sealed vessel at120° C. for 2 hours to thereby decompose the oxymethylene copolymerresin. After cooling the resultant reaction mixture, the amount ofalkylene glycol present therein is measured by gas chromatography (bymeans of a flame ionization detector, i.e., FID), to determine theamount of oxyalkylene monomer units. The amount of oxyalkylene monomerunits is expressed in terms of mol %, based on the oxymethylene monomerunits.

(2) Weight decrease ratio R (%) as measured by heating an oxymethylenecopolymer resin at a temperature of 230° C. for 100 minutes in anitrogen atmosphere:

1 g of an oxymethylene copolymer resin is placed in a 10 ml glass flaskequipped with a nitrogen introduction tube. Then, the flask containingthe copolymer resin is immersed in an oil bath at 230° C. and allowed tostand therein for 100 minutes while introducing nitrogen at a rate of 10liters/hour. After that period of time, the weight (g) of theoxymethylene copolymer resin is measured. The weight decrease ratio R ofthe oxymethylene copolymer resin is defined by the formula ##EQU2##wherein W₁ is the weight of the oxymethylene copolymer resin before theheating, and W₂ is the weight of the oxymethylene copolymer resin afterthe heating.

(3) Flexural modulus:

An oxymethylene copolymer resin composition is molded by means of aninjection molding machine (IS-100E-3A, manufactured and sold by ToshibaKikai Co., Japan) under the following conditions.

Clamping pressure: 100 ton

Injection pressure: 700 kgf/cm²

Injection rate: 5 cm/sec

Injection time (sec)/cooling time (sec): 10/5

Cylinder temperature: 210° C.

Revolution number of screws: 100 rpm

Mold temperature: 40° C.

With respect to the obtained shaped article, a flexural modulus ismeasured in accordance with ASTM-D-790.

(4) Amount ratio of terminal formate groups (D₁₇₁₀ /D₁₄₇₀):

An oxymethylene copolymer resin is subjected to hot pressing at 200° C.to thereby obtain a film having a thickness of 15 μm. An infraredabsorption spectrum of the film is obtained. From the infraredabsorption spectrum, the ratio of an absorbance at the wave number of1710 cm⁻¹ to an absorbance at the wave number of 1470 cm⁻¹, i.e., D₁₇₁₀/D₁₄₇₀ ratio is calculated.

(5) Thermal stability of an oxymethylene copolymer resin composition inan oxygen-containing atmosphere:

A weight decrease of an oxymethylene copolymer resin composition ismeasured by means of a thermobalance. Specifically, an oxymethylenecopolymer resin composition is kept at 230° C. in air, using athermogravity analyzer (TGA-7 manufactured and sold by Perkin-Elmer,U.S.A.), while measuring a decrease in weight. In terms of the period oftime required for the weight to decrease by 20%, based on the originalweight, the thermal stability of the oxymethylene copolymer resincomposition in an oxygen-containing atmosphere is evaluated. The longerthe above-mentioned period of time, the higher the thermal stability inan oxygen-containing atmosphere.

REFERENTIAL EXAMPLE 1 Production of Oxymethylene Copolymer Resins

(1) Production of sample A:

2000 g of highly purified trioxane (water content: 2 ppm; formic acidcontent: 3 ppm), 1,3-dioxolan (0.8 mol %, based on the trioxane) andmethylal (0.2 mol %, based on the trioxane) were charged into a kneaderhaving 2 sigma blades and a jacket, and the internal temperature of thekneader was elevated to 70° C. Then, a (0.002 mol/liter) cyclohexanesolution of boron trifluoride dibutyl ether was added in an amount suchthat its final concentration became 0.15×10⁻⁴ mol per mol of the totalof the trioxane and 1,3-dioxolan, and a reaction was conducted. 30minutes after the start of the reaction, 1 liter of an aqueous 30%triethylamine solution was added to thereby deactivate the catalyst andterminate the reaction. The obtained reaction mixture was furtherkneaded for 1 hour. Then, the content of the kneader was taken out andsubjected to filtration, thereby obtaining an oxymethylene copolymerresin to be subsequently treated for stabilizing terminals thereof(hereinafter, frequently referred to as "crude oxymethylene copolymerresin"). The obtained crude oxymethylene copolymer resin was dried at100° C. 100 parts by weight of the crude oxymethylene copolymer resinwas mixed with 3 parts by weight of an aqueous triethylamine solution(composed of 0.5 part by weight of triethylamine and 2.5 parts by weightof water) and 0.2 part by weight of2,2-methylenebis-(4-methyl-6-t-butylphenol) and the resultant mixturewas subjected to extrusion, using a vented, single-screw extruder(residence time: 300 seconds), to thereby change unstable terminals ofthe molecules to stable terminals, thereby obtaining an oxymethylenecopolymer resin (sample A)(extrusion temperature: 200° C.; ventpressure: 200 torr). The obtained oxymethylene copolymer resin had amelt index of 9.5 g/10 min as measured at 190° C. in accordance withASTM D1238.

(2) Production of samples B to G:

Substantially the same procedure as in item (1) above was repeatedexcept that the type and amount of the comonomer were varied, therebyobtaining 6 types of oxymethylene copolymer resins (samples B to G). Thetype and amount of the comonomer employed for producing samples B to Gare indicated in Tables 1 and 2. All of the obtained 6 types ofoxymethylene copolymer resins had a melt index of 9.5 g/10 min asmeasured at 190° C. in accordance with ASTM D1238.

With respect to each of samples A to G, the amount ratio of terminalformate groups (D₁₇₁₀ /D₁₄₇₀), the amount of oxyalkylene monomer unitsinserted and the weight decrease ratio R as measured by heating theoxymethylene copolymer resin at a temperature of 230° C. for 100 minutesin a nitrogen atmosphere are shown in Tables 1 and 2.

REFERENTIAL EXAMPLE 2 Production of Poly-β-alanines

4.5 kg of acrylamide, 0.5 kg of N,N'-methylenebisacrylamide and 1.008 gof zirconium tetraisopropoxide (the molar amount of the zirconiumtetraisopropoxide was 1/20,000 of the total molar amount of themonomers) were charged into a Werner-type, 30 liter batch kneader(manufactured and sold by Yoshida Seisakusho, Japan), and reacted at125° C. for 4 hours. The resultant solid reaction mixture was taken outof the kneader and pulverized. In the pulverization, the pulverizationtime and force were varied so that various poly-β-alanines havingdifferent average particle diameters (for example, 2.5 μm) as indicatedin Tables 1 and 2, were obtained.

EXAMPLES 1 to 14

Samples A to E were individually mixed with a poly-β-alanine and a basicsubstance as shown in Table 1, and the resultant mixture wasmelt-kneaded in a twin-screw extruder having a cylinder temperature setat 200° C., and the kneaded mixture was pelletized, thereby obtaining apelletized oxymethylene copolymer resin composition. With respect toeach of the obtained oxymethylene copolymer resin compositions, the timerequired for the weight of the composition to decrease by 20% (based onthe original weight when heated at 230° C. in an oxygen-containingatmosphere), and the flexural modulus measured with respect to theshaped articles produced from the above-mentioned oxymethylene copolymerresin compositions by the method described in item (3) above! are shownin Table 1. As apparent from Table 1, the obtained oxymethylenecopolymer resin compositions had high mechanical properties and thermalstability in an oxygen-containing atmosphere.

COMPARATIVE EXAMPLE 1

A composition was produced in substantially the same manner as inExample 1 except that a commercially available oxymethylene homopolymer(Tenac 5000, manufactured and sold by Asahi Chemical Industry Co., Ltd.,Japan) (terminal groups: acetyl groups) was mixed with a poly-β-alanineindicated in Table 2. Next, various measurements were conducted in thesame manner as in Example 1. Results are shown in Table 2. Theoxymethylene homopolymer obtained had terminal acetyl groups, which wereeasily decomposed by heat to produce a large amount of formaldehyde. Theformed formaldehyde underwent oxidation to form formic acid, so that theoxymethylene homopolymer resin composition had poor thermal stability inan oxygen-containing atmosphere.

COMPARATIVE EXAMPLE 2

A composition was produced in substantially the same manner as inExample 1, except that a commercially available oxymethylene copolymerresin (Tenac 4520, manufactured and sold by Asahi Chemical Industry Co.,Ltd., Japan) (the amount of oxyalkylene monomer units inserted: 1.6 mol%) was mixed with a poly-β-alanine indicated in Table 2. Next, variousmeasurements were conducted in the same manner as in Example 1. Resultsare shown in Table 2. The obtained oxymethylene copolymer resincomposition was poor in mechanical properties.

COMPARATIVE EXAMPLE 3

A composition was produced in substantially the same manner as inExample 1, except that an oxymethylene copolymer resin (sample F) (theamount of oxyalkylene monomer units inserted: 0.03 mol %) was mixed witha poly-β-alanine indicated in Table 2. Next, various measurements wereconducted in the same manner as in Example 1. Results are shown in Table2. The treatment of the oxymethylene copolymer resin for changing itsunstable terminals to stable terminals could not be satisfactorilyconducted. As a result, the amount of formaldehyde formed by a heatdecomposition of the unstable terminals of the oxymethylene copolymerresin was large. The formed formaldehyde underwent oxidation to formformic acid, so that the oxymethylene copolymer resin composition hadpoor thermal stability in an oxygen-containing atmosphere.

COMPARATIVE EXAMPLE 4

A composition was produced in substantially the same manner as inExample 1, except that an oxymethylene copolymer resin (sample G) (theamount of oxyalkylene monomer units inserted: 0.8 mol %) was mixed witha poly-β-alanine indicated in Table 2. Next, various measurements wereconducted in the same manner as in Example 1. Results are shown in Table2. The obtained oxymethylene copolymer resin composition was poor inmechanical properties.

COMPARATIVE EXAMPLE 5

Various measurements were conducted in the same manner as in Example 1with respect to an oxymethylene copolymer resin (sample A) (the amountof oxyalkylene monomer units inserted: 0.2 mol %) which was not mixedwith a poly-β-alanine. Results are shown in Table 2. Because theoxymethylene copolymer resin was not mixed with a poly-β-alanine, theamount of formaldehyde formed by a heat decomposition was large, and theformed formaldehyde underwent oxidation to form formic acid, so that theoxymethylene copolymer resin had poor thermal stability in anoxygen-containing atmosphere.

COMPARATIVE EXAMPLE 6

A composition was produced in substantially the same manner as inExample 1, except that an oxymethylene copolymer resin (sample A) (theamount of oxyalkylene monomer units inserted: 0.2 mol %) was mixed with0.003 part by weight of a poly-β-alanine indicated in Table 2. Variousmeasurements were conducted in the same manner as in Example 1. Resultsare shown in Table 2. Because the amount of the poly-β-alanine was toosmall, the formaldehyde formed by a heat composition was notsatisfactorily captured, and the formed formaldehyde underwent oxidationto form formic acid, so that the oxymethylene copolymer resincomposition had poor thermal stability in an oxygen-containingatmosphere.

COMPARATIVE EXAMPLE 7

A composition was produced in substantially the same manner as inExample 1, except that an oxymethylene copolymer resin (sample A) (theamount of oxyalkylene monomer units inserted: 0.2 mol %) was mixed with5 parts by weight of a poly-β-alanine indicated in Table 2. Variousmeasurements were conducted in the same manner as in Example 1. Resultsare shown in Table 2. Because the amount of the poly-β-alanine was toolarge, a shaped article made from the produced composition becameyellowish by discoloration.

COMPARATIVE EXAMPLE 8

A composition was produced in substantially the same manner as inExample 1, except that an oxymethylene copolymer resin (sample A) (theamount of oxyalkylene monomer units inserted: 0.2 mol %) was mixed with0.3 part by weight of a poly-β-alanine (having an average particlediameter of 30 μm) indicated in Table 2. Various measurements wereconducted in the same manner as in Example 1. Results are shown in Table2. The formaldehyde formed by a heat decomposition could not besatisfactorily captured, and the formed formaldehyde underwent oxidationto form formic acid, so that the oxymethylene copolymer resincomposition had poor thermal stability in an oxygen-containingatmosphere.

                                      TABLE 1    __________________________________________________________________________                                    Weight decrease                                    ratio R (%) as                        Amount of                              Amount ratio                                    measured by heating                        oxyalkylene                              of terminal                                    oxymethylene                        monomer                              formate                                    copolymer resin          Oxymethylene  units groups                                    at 230° C. for 100          copolymer     inserted                              (D.sub.1710 /                                    minutes in nitrogen          resin  Comonomer                        (mol %)                              D.sub.1470)                                    atmosphere    __________________________________________________________________________    Example 1          Sample A                 1,3-dioxolane                        0.2   0.015 0.8    Example 2          Sample B                 1,3-dioxolane                        0.1   0.017 1.5    Example 3          Sample C                 1,3-dioxolane                        0.4   0.013 0.4    Example 4          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 5          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 6          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 7          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 8          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 9          Sample E                 1,4-butanediol                        0.1   0.020 1.6                 formal    Example 10          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 11          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 12          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 13          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    Example 14          Sample D                 1,3-dioxolane                        0.15  0.018 1.0    __________________________________________________________________________                                Time required for weight                                of oxymethylene copolymer    Poly-β-alanine         resin composition to decrease    Average       Basic substance                                by 20%, based on the original         particle              Amount   Amount                           Flexural                                weight, at 230° C. in an         diameter              (part by (part by                           modulus                                oxygen-containing atmosphere         (μm)              weight)                  Type weight)                           (kg/cm.sup.2)                                (min)    __________________________________________________________________________    Example 1         2.5  0.2 --   --  29,000                                100    Example 2         2.5  0.2 --   --  30,000                                80    Example 3         2.5  0.2 --   --  28,000                                150    Example 4         2.5  0.2 --   --  29,000                                90    Example 5         2.5  0.02                  --   --  29,000                                60    Example 6         2.5  0.5 --   --  29,000                                180    Example 7         5    0.2 --   --  29,000                                60    Example 8         1    0.2 --   --  29,000                                130    Example 9         2.5  0.2 --   --  30,000                                80    Example         2.5  0.2 Calcium                       0.05                           29,000                                180    10            stearate    Example         2.5  0.2 Calcium                       0.5 29,000                                240    11            stearate    Example         2.5  0.2 Melamine                       0.1 29,000                                120    12    Example         2.5  0.2 Methylol                       0.1 29,000                                120    13            melamine    Example         2.5  0.2 Hydro-                       0.01                           29,000                                110    14            talcite    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________                                     Weight decrease                                     ratio R (%) as                        Amount of    measured by heating                        oxyalkylene                              Amount ratio                                     oxymethylene                        monomer                              of terminal                                     copolymer resin          Oxymethylene  units formate                                     at 230° C. for 100          copolymer     inserted                              groups minutes in nitrogen          resin  Comonomer                        (mol %)                              (D.sub.1710 /D.sub.1470)                                     atmosphere    __________________________________________________________________________    Comparative          Oxymethylene                 --     --    Acetyl 6.0    Example 1          homopolymer         group    Comparative          Oxymethylene                 Ethylene oxide                        1.6   0.018  0.1    Example 2          copolymer    Comparative          Sample F                 1,3-dioxolane                        0.03  0.018  3.5    Example 3    Comparative          Sample G                 1,3-dioxolane                        0.8   0.013  0.2    Example 4    Comparative          Sample A                 1,3-dioxolane                        0.2   0.015  0.8    Example 5    Comparative          Sample A                 1,3-dioxolane                        0.2   0.015  0.8    Example 6    Comparative          Sample A                 1,3-dioxolane                        0.2   0.015  0.8    Example 7    Comparative          Sample A                 1,3-dioxolane                        0.2   0.015  0.8    Example 8    __________________________________________________________________________                                 Time required for weight                                 of oxymethylene copolymer    Poly-β-alanine          resin composition to decrease          Average  Basic substance                                 by 20%, based on the original          particle               Amount   Amount                            Flexural                                 weight, at 230° C. in an          diameter               (part by (part by                            modulus                                 oxygen-containing atmosphere          (μm)               weight)                   Type weight)                            (kg/cm.sup.2)                                 (min)    __________________________________________________________________________    Comparative          2.5  0.2 --   --  30,800                                 10    Example 1    Comparative          2.5  0.2 --   --  23,000                                 200    Example 2    Comparative          2.5  0.2 --   --  30,000                                 40    Example 3    Comparative          2.5  0.2 --   --  25,000                                 180    Example 4    Comparative          --   --  --   --  29,000                                 15    Example 5    Comparative          2.5  0.003                   --   --  29,000                                 15    Example 6    Comparative          2.5  5   --   --  29,000                                 250    Example 7    Comparative          30   0.3 --   --  29,000                                 30    Example 8    __________________________________________________________________________

INDUSTRIAL APPLICABILITY

The oxymethylene copolymer resin composition of the present inventionexhibits not only mechanical properties which are comparable or superiorto the mechanical properties of conventional oxymethylene polymercompositions as well as conventional oxymethylene polymers, but alsoexhibits an excellent thermal stability which has not conventionallybeen achieved. Therefore, the oxymethylene copolymer resin compositionof the present invention can be advantageously used as materials forautomobile parts, electrical parts, etc.

We claim:
 1. An oxymethylene copolymer resin composition comprising:(A)100 parts by weight of an oxymethylene copolymer resin comprising aplurality of oxymethylene copolymer chains, each comprising recurringoxymethylene monomer units and oxyalkylene monomer units insertedtherein, wherein said oxyalkylene monomer units, each having at least 2carbon atoms, are present in said oxymethylene copolymer resin in anamount of from 0.05 to 0.5 mol %, based on said oxymethylene monomerunits,said plurality of oxymethylene copolymer chains collectivelyhaving, as terminal groups, alkoxyl groups each having at least onecarbon atom, hydroxyalkyl groups each having at least 2 carbon atoms,and formate groups; (B) 0.01 to 3.0 parts by weight of a poly-β-alaninehaving an average particle diameter of 6 μm or less; and (C) 0.05 to 0.6part by weight, per 100 parts by weight of said oxymethylene copolymerresin, of an alkaline earth metal salt of an organic fatty acid.
 2. Theoxymethylene copolymer resin composition according to claim 1, whereineach oxyalkylene monomer unit independently has 2 to 10 carbon atoms,each terminal alkoxyl group independently has 1 to 10 carbon atoms, andeach terminal hydroxyalkyl group independently has 2 to 10 carbon atoms.3. The oxymethylene copolymer resin composition according to claim 1,wherein said oxymethylene copolymer resin (A) exhibits a weight decreaseratio R of 3% or less as measured by heating said oxymethylene copolymerresin at a temperature of 230° C. for 100 minutes in a nitrogenatmosphere, said weight decrease ratio R being defined by the formula:##EQU3## wherein W₁ is the weight of the oxymethylene copolymer resinbefore the heating, and W₂ is the weight of the oxymethylene copolymerresin after the heating.
 4. The oxymethylene copolymer resin compositionaccording to claim 1, 2 or 3, wherein said oxymethylene copolymer resin(A) exhibits in the infrared absorption spectrum thereof an absorbanceD₁₇₁₀ ascribed to the terminal formate groups at the wave number of 1710cm⁻¹ and an absorbance D₁₄₇₀ ascribed to the oxymethylene groups at thewave number of 1470 cm⁻¹, wherein D₁₇₁₀ and D₁₄₇₀ satisfy therelationship defined by the formula:

    D.sub.1710 /D.sub.1470 ≦0.025.